In a current computed tomography system, an x-ray source emits a fan-shaped beam which is collimated to lie within an X-Y plane of a Cartesian coordinate system, termed the “imaging plane.” The x-ray beam passes through the object being imaged, such as a medical patient, and impinges upon an array of radiation detectors. The intensity of the transmitted radiation is dependent upon the attenuation of the x-ray beam by the object and each detector produces a separate electrical signal that is a measurement of the beam attenuation. The attenuation measurements from all the detectors are acquired separately to produce the transmission profile.
The source and detector array in a conventional CT system are rotated on a gantry within the imaging plane and around the object so that the angle at which the x-ray beam intersects the object constantly changes. A group of x-ray attenuation measurements from the detector array at a given angle is referred to as a “view” and a “scan” of the object comprises a set of views made at different angular orientations (θ) during one revolution of the x-ray source and detector. In a 2D scan, data is processed to construct an image that corresponds to a two dimensional slice taken through the object. The prevailing method for reconstructing an image from 2D data is referred to in the art as the filtered backprojection technique. This process converts the attenuation measurements from a scan into integers called “CT numbers” or “Hounsfield units”, which are used to control the brightness of a corresponding pixel on a display.
The in-plane resolution of a CT image is determined by the sampling density of the x-ray projection views over the subject of the scan. More specifically, resolution is determined by the density of the projection view angles acquired during the scan and the sampling density of the detector elements in each acquired projection view. A number of techniques are known for increasing image resolution over that which is possible from a given detector array and a given number of acquired views. These include the so-called quarter-detector-shift scans and focal spot wobbling techniques, which both require hardware modifications to the CT systems.
Image resolution can be increased by reducing the size of the detector elements to increase detector resolution. Current systems employ detector elements that are 1 mm size and by using quarter-detector-shift and focal spot wobbling the image resolution may be increased to 0.5 mm in the in-plane dimensions. Detector elements can be manufactured to be smaller and to thus enable a higher density, higher resolution detector array to be constructed.
However, to maintain a sufficient image SNR, the x-ray dose must be increased and the cost of the detector array is significantly increased. As a result, there is a practical limit to increasing image resolution by reducing detector element size.
The same resolution limitations apply to three-dimensional CT. A 3D volume CT employs an x-ray source that emits a cone beam on a two-dimensional array of detector elements. Each acquired view is thus a 2D array of x-ray attenuation measurements and a complete scan is performed by acquiring multiple views as the x-ray source and detector array are revolved around the subject to produce a 3D array of attenuation measurements. Both the in-plane and axial resolution of the image that can be produced is determined by the detector resolution.